The mechanical pump comprises a pump rotor with radially slidable vanes rotating in a shiftable control ring, whereby the control ring is pushed by a first plunger pushing the control ring into high pumping volume direction. The pump further comprises a pressure control system for controlling the discharge pressure of the pressurized lubricant, whereby the control system comprises a first pressure control chamber wherein the first plunger is provided axially moveable. The pump is provided with a first pressure conduit which connects a pump outlet port with the first control chamber. The pump is also provided with a first pressure control valve which controls the pressure in the first pressure control chamber.
Variable displacement vane pumps of the state of the art are described in WO 2005/026553 A1. The pump is provided with a pressure control system for controlling the discharge pressure of the lubricant. The pressure control system comprises a first pressure control chamber wherein a first plunger is provided being axially movable. The first pressure control chamber is connected via a first pressure conduit with the pump outlet port. The pressure control system also comprises a separate control element which is realized as a cylinder-piston-element which keeps the pressure of the pressurized lubricant provided by the pump at a more or less constant level, independent of the rotational speed of the pump rotor. This is realized by opening and closing a control outlet of the first pressure control chamber, thereby moving the control ring into a low pumping volume direction or pushing into a high pumping volume direction.
The lubricant pressure requirement is, however, dependent on the working conditions of the engine. The lubricant pressure requirement is lower at low rotational speeds and higher at high rotational speeds of the engine.
A device with a control system which provides two levels of pressure of the pressurized lubricant is described in DE 10 2004 049 029 A1. The pressure control of the device is performed by using an electromagnetic valve. Electromagnetic valves are not, however, fail-safe, so that an electrical power loss can lead to an insufficient lubrication of the engine.